1. Technical Field
The invention relates generally to measuring visibility, and more particularly, to an image-based solution for measuring visibility.
2. Background Art
Visibility, or visual range, is the distance at which a target object of a given size and characteristics can be seen. Knowledge of visibility is important for transportation, meteorological, and environmental applications. For example, visibility can be used to monitor safety conditions and/or impose a speed limit along a waterway, highway, or the like. Similarly, visibility is important in take off and landing aircraft, monitoring weather conditions like fog and precipitation, and monitoring haze and air pollution in industrial areas and wildlife reserves.
In general, the visibility of a target object is dependent on its “contrast”. Contrast is defined as the ratio of the difference of the luminance of the target object and its background to the luminance of the background. A target object having a contrast that exceeds a particular threshold, generally 0.02, is considered visible. To this extent, the visual range can be defined as the distance from a target object at which the contrast of the target object drops to 0.02.
With respect to visibility, the “extinction coefficient”, or attenuation coefficient, is a measure of an amount of visibility loss over a given distance. Extinction is caused by two major factors. The first is light scattering, which is the deflection of light from one path into another, and is caused, for example, by some atmospheric components. Light scattering contributes to extinction by scattering light from a target object out of the line of vision, thereby reducing the total incoming light from the target, and by scattering light from other directions into the line of vision, thereby adding spurious light signatures to those received from the target. The second primary cause of extinction is light absorption, which reduces the light available to arrive from the target object, and is caused, for example, by other components of the atmosphere.
Historically, a lack of visibility, e.g., due to heavy rain, fog, snow, or the like, has contributed to more accidents in the various travel modalities, and particularly air travel, than virtually any other cause. As a result, a number of solutions have been proposed to evaluate and/or monitor visibility conditions. For example, a transmissometer measures fluctuations in light received from a known source. Alternatively, a nephelometer measures light scattering based on the light scattered by an aerosol. Additionally, a scattered light meter estimates the total amount of scattered light that is received, and a teleradiometer, or light meter, measures the total brightness of a particular target object.
However, each of these solutions is limited. For example, the transmissometer requires widely separated, precisely aligned components, and fails to account for extra light that is scattered into the field of view. The nephelometer and scattered light meter do not measure the loss of light due to absorption, and the teleradiometer is limited to measuring the amount of light that arrives from the bearing of the target and is sensitive to non-uniform lighting and variations in inherent contrast.
In order to compensate for one or more non-measured components in these solutions, model-based calculations and/or a combination of multiple solutions into a single package have been proposed. However, even alone, these solutions are expensive, often large, and the results provided do not always accord with visual estimates provided by trained observers. In particular, most commercial visibility sensors determine scattering properties within a relatively small air volume and use a transfer algorithm to convert the observation into a distance measurement. However, the measurements are based on the clarity of the air, and not the distance at which an individual can see a target object. To this extent, the same air quality may correspond to a wide range of visibilities based on the target object, lighting condition, and the like.
Other visibility measurement solutions propose the use of a video-based measurement approach. Such an approach offers a number of potential advantages, such as similarity to what is seen by a human, highly quantitative image data, as well as the availability of large image databases that can be used to prototype a solution from a set of known standardized materials. However, current video-based solutions are limited by requiring specialized and standardized target objects for measurement, require expensive, fragile equipment, provide variable results based on the time of day, lighting, and other factors, and require power consumption that makes deployment outside of an urban or suburban area questionable, if not impossible.
Scattering is characterized by the scattering coefficient, S, while absorption is characterized by the absorption coefficient, A. To this extent, the extinction coefficient, α, is given by:α=S+A. Additionally, an inherent contrast, C0, of the target object is the contrast of the target object at a zero distance. Based on the Koschmieder relationship, at a distance, r, the corresponding contrast, Cr, can be calculated by:Cr=C0e−α.This equation makes several assumptions: (1) that the extinction coefficient does not vary in the horizontal direction, i.e., that the atmosphere is homogenous; (2) that the background is the sky; and (3) that the sky radiance is the same at the target object as it is at the observer.
Using a black target object (e.g., C0=−1), visibility, V, can be determined as the distance, r, at which Cr reduces to 0.02. This yields the equation:
  V  =                    ln        ⁡                  (                      1            /            0.02                    )                    α        =          3.912      α      A transmissometer and/or nephelometer can be used to measure the extinction coefficient, a, and the equation above can be used to derive the visibility. Alternatively, the extinction coefficient can be determined by placing two black target objects at different distances, r1 and r2, from an observation point. Using a telephotometer, the contrasts, C1 and C2, for the respective black target objects from the observation point can be determined. Subsequently, the extinction coefficient can be calculated by:
  α  =                                  ln          ⁢                                          ⁢                      C            1                          -                  ln          ⁢                                          ⁢                      C            2                                                r          1                -                  r          2                        
By directly or indirectly assuming that the decay of contrast follows the Koschmeider equation, existing solutions have several weaknesses. In particular, the Koschmeider equation assumes that the background is the sky and that the extinction coefficient remains constant along the path of vision. Neither of these assumptions is valid. For example, in determining the visibility of an aircraft pilot, clouds may be present and/or the pilot may be looking downward towards a prospective landing spot, the background of which is the ground. Further, the visual range may vary from region to region. As a result, a single measurement of the extinction coefficient will fail to account for these differences. For example, a fogbank may obscure a freeway for a short distance, while the rest of the field of view remains clear. Still further, the contrast of a target object, in terms of luminance, may vary significantly due to variations in sunlight, shadows, cloud cover, etc., while an observer's perception of the visibility of the target object remains substantially the same. Still further, a human will take into account other factors, such as the sharpness of features, clarity of texture, distinctiveness of color, and the like, when determining the target object's visibility.
To this extent, a need exists for an improved image-based visibility measurement solution that is not limited by one or more of these weaknesses.